1. Field of the Invention
The present invention relates to a high-frequency filter for use in radio communication equipment, measuring equipment or the like, and more particularly to a high-frequency filter having an excellent mechanical strength.
2. Description of the Prior Art
In recent years, high-frequency filters have been used widely in the field of radio communication equipment, such as car telephones, portable telephones or personal radio communications. Conventional high-frequency filters include a dielectric board which is, in general, made of alumina or polytetrafluoroethylene (PTFE) fiber reinforced glass, as disclosed in Japanese Laid-open Patent Publication No. 60-114001 entitled "Coaxial Dielectric Filter".
However, the disclosed high-frequency filter has a drawback that as shown in FIGS. 8A and 8B of the accompanying drawings, the dielectric board 1 is likely to cause a break of coupled circuits due to a damage at the junction between the dielectric board 1 and input/output pins 2 or between the dielectric board 1 and central conductors 3.
In particular, when the dielectric board 1 is made of a dielectric ceramic such as calcined alumina or barium titanate, it becomes likely that due to a pressure or force applied via the input/output pins 2, microcracks produced in the vicinity of the input/output pins 2 or holes 4 receiving therein the central conductors 3 are enlarged and eventually bring about fracture of the dielectric board 1. This is because the ceramic constituting the dielectric board 1 is a brittle material which is capable of absorbing only a little energy before its fracture. As against the brittleness, the toughness is represented in terms of the critical stress intensity factor (or fracture toughness) K.sub.1c. The K.sub.1c of ceramic is in the range of 2-4 MPa.multidot.m.sup.1/2. The brittleness of the ceramic is remarkable because even cast iron which is the most brittle material among ferrous metals has a K.sub.1c value of about 20 Mpa.multidot.m.sup.1/2. In FIGS. 8A and 8B, numeral 5 designates resonators and 6 is a case for holding therein the resonators 5 and the dielectric board 1.
In the case where the dielectric board 1 is a plastic board made, for example, of polytetrafluoroethylene (PTFE) fiber reinforced glass, the foregoing problem of board fracture does not arise due to a high resiliency of the plastic board. However, the plastic board has a drawback that the moisture resistance (tan. .delta., in particular) and heat resistance are poor as compared to those of the ceramic board.